Birch Trees - Insects and Disease

Shelley:I'm with UW-Extension Entomologist Phil Pellitteri. We're talking about Birch trees. Phil, I just saw a gorgeous Weeping Birch that I would love to have. I was told that it's called a "suicide tree." Why is that?

Phil:It's a high-maintenance tree because of some insect problems. We find that there is an insect in the southern part of the state in particular called the Bronze Birch Borer that attacks birches. And a lot of it has to do with where we plant them. Birch is not a prairie tree.

Shelley:Oh, darn!

Phil:A lot of people put it out by itself. In the northern part of the state, where it's a native tree, it's with other birches where the shade helps it out a lot, or down by the lake, where it's nice and cool. The problem we get here is when we plant it out in exposed sites; it's a very shallow rooted tree, and it gets under moisture stress. Once it starts to get stressed it literally gives off chemicals that attract the borer.

Shelley:Wow, so it really kills itself almost. It attracts something that deadly.

Phil:The insect is called the Bronze Birch Borer. It typically kills the tree in thirds. So, when you drive around, you'll often see a lot of trees that have the top third of the tree starting to go out...

Shelley:Like this one here.

Phil:This is a classic example. Often, it will take two or three years to die. In a bad planting site where it's highly stressed, it might die the first year.

Shelley:Well, let's look at it; we've got it right here. What would we look for if we're trying to I.D. it?

Phil:Other than the thin leaves and the die-back going on, when you look closely at the bark, you see symptoms. One of the symptoms is a "D" shaped emergence hole. This one is a little unusual in that it's weeping, which is the way the tree is trying to defend itself and keep the insect out. So, this shows us the tree has got some strength. Typically, what you will see, though, instead is just the "D" shaped hole. Underneath this, the insects, in mature stage, is girdling the tree. Basically, it's cutting it so everything above that point dies.

Shelley:So, no nutrients are getting to the top.

Phil:Right, so it's easy to see it. They progress down the tree given the right chance.

Shelley:So, the first thing to do is plant it in the right spot.

Phil:Right. The north side is much better than the south side. Anything you can do to cool the roots of the tree, like putting plantings down, you know, junipers. Mulch will help. But water the tree, even maybe weekly would be a good idea. And that's the mistake-- Even with trees in a marginal site, often it's sudden. The drought comes, you know, every three or four years. That allows the insect to get a foothold.

Shelley:Okay, what do we do about it?

Phil:Other than strengthening the tree any way you can, there are some systemic treatments that can be used whether you call an arborist to put it on-- Or, you can buy some stuff, if you buy the right stuff. Soak it into the ground, and that will bring it up. But I think, probably, a better thing is more of a preventative side. Talking to the local nursery people on what's a proper planting site and varieties. Some of the good news is there are cultivars, like Whitespire and Heritage, that have some resistance. There'll be new varieties developed.

Shelley:River Birches rather than the Paper Birches?

Phil:Definitely. You see the weeping birches are the worst. The White Canoe Birches are kind of in the middle. River Birch, in particular, are the least susceptible. Often, you can put those in a reasonably stressful site and not have many problems.

Shelley:Right site, right plant is the first step. Are there other problems or is this the main issue?

Phil:We've had a cosmetic problem called a Birch Leaf Miner. It's a little insect that lays her eggs and the larvae develop internally. If you look at these blotches carefully, you'll see kind of a blackish, pepper-like residue, which is left behind by the insect. In many years, we'd see 70 or 80 percent of the leaves infested. We're considering that a stress factor that would maybe allow the borer to get on. We've often treated this preventatively because it was an annual problem. What's so interesting is the last three years in the southern part of the state, this has become a very infrequently found insect. We had to search hard for these. One of the things that we're suggesting now is not to treat preventatively. If the insect isn't there, there's no reason to treat. Until it shows up again, there's no reason to justify these spring treatments.

Shelley:This is purely cosmetic. But in heavy doses, it led to stress that then brought in the Bronze Birch Borer.

Phil:Any of those stress factors are a concern.

Shelley:Thanks, Phil. So, the real trick is to pick the right birch and to plant it in the right spot.

Join Olbrich Botanical Garden Director of Horticulture, Jeff Epping, as he shows off a collection of smaller willows. He offers tips on how to prune the trees so they can be grown in small yards.

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Birches Go Beyond LandscapingBirch trees provide more than beauty and shade. The root is used to make birch beer, the syrup can be used like maple syrup. The bark has been used to make canoes and containers. Recently, betulinic acid, found in the bark, has been found to be useful against melanoma cancer and HIV disease.

Bug Off: Insects and Diseases of Birch Trees UW-Extension Entomologist Phil Pellitteri discussed why it's important to choose the right birch tree for the landscape to avoid problems with insects and disease. The lovely European weeping birch described above is one that is quite susceptible to bronze birch borer (Agrilus anxius), a buprestid beetle that bores into the trunk and eventually kills it. It is particularly devastating on weeping birch, European white birch or cut-leaf varieties (all types of Betula pendula) and yellow birch (B. alleghaniensis). The telltale signs of infestation are "D" shaped exit holes in the trunk and branches, and the tree will quickly die back from the top down.

However, with proper management you can prevent these particular varieties from succumbing to borers. Birches are shallow-rooted, northern zone trees, and need to have their roots remain cool and moist. If the roots get dry, the stressed tree emits a chemical that attracts the bronze birch borer. To prevent this, plant the tree in the right spot, especially in southern Wisconsin, in a north exposure or a cool shaded site. If you already have a birch planted in the wrong site you can help minimize stress by mulching around the base, planting other plants around the base to help keep the soil cool, and watering more.

An even easier way to avoid bronze birch borer is to plant a resistant variety. The most resistant birch is currently a river birch (B. nigra), but Whitespire birch (B. platyphylla var. japonica 'Whitespire') also has some resistance. 'Heritage' is a relatively white-barked cultivar of river birch. The U.S. National Arboretum and others have breeding programs to develop resistant birches with desirable landscape characteristics. There are new resistant cultivars coming out all the time, so shopping at a reputable nursery will put you in touch with the newest, resistant varieties.